1. Field of the Invention
This invention relates to nonlinear optical systems, and particularly to substituted stilbenes capable of second harmonic generation (SHG) and having other useful nonlinear optical and electro-optic properties.
2. Description of Related Art
The nonlinear optical response of a molecule can be described by the following expansion: EQU .mu.=.mu..sub.0 +.alpha.E+.beta.EE+.lambda.EEE+. . .
where .mu. is the induced dipole moment and .mu..sub.0 is the permanent dipole moment of the molecule; .alpha., .beta., and .lambda. are the linear, second order and third order polarizabilities, respectively; E is the applied electric field. To describe an ensemble of molecules such as a crystal, the macroscopic relationship should be used: EQU P=P.sub.0 +.chi..sup.(1) E+.chi..sup.(2) EE+.chi..sup.(3) EEE+. . .
where P is the induced polarization and P.sub.0 is the permanent polarization; .chi..sup.(1), .chi..sup.(2) and .chi..sup.(3) are the linear, second order and third order susceptibility, respectively. Second order nonlinear optical phenomena such as second harmonic generation (SHG), sum and difference frequency generation, parametric processes and electro-optical effects all arise from the .chi..sup.(2) term. To have a large .chi..sup.(2), a molecule should both possess a large .beta. and crystallize in a noncentrosymmetric structure. Centrosymmetric crystals have vanishing .chi..sup.(2) and are therefore incapable of SHG.
Franken, et al., Physical Review Letters, Vol. 7, 118-119 (1961), disclose the observation of SHG upon the projection of a pulsed ruby laser beam through crystalline quartz. They observed the generation of the second harmonic of light, in which light of 6943 .ANG. was converted to light of 3472 .ANG.. The use of a laser remains the only practical way to generate an E large enough to be able to detect the SHG phenomenon.
Coda et al., J. Appl. Cryst., Vol. 9, 193 (1976), disclose SHG in a powder sample of 4-methoxy-4'-nitrostilbene.
Kurihara, et al., J. Chem. Soc., Chem. Commun., 959-960 (1987), disclose the synthesis of 4-methoxy-4'-nitrotolan (MNT) (i.e., 4-methoxy-4'-nitrodiphenylacetylene) and the use of MNT for SHG.
Fouquey, et al., J. Chem. Soc. Chem. Commun., 1424-6 (1987), disclose the preparation and crystal phase transition temperatures for several 4-amino-4'-nitrostilbene and 4-nitrodiphenylacetylene derivatives. Non-linear optical properties, including second harmonic generation, are noted for selected compounds.
Useful reviews of the art relating to nonlinear properties of organic materials are given in the following references: "Nonlinear Optical Properties of Organic and Polymeric Materials", D. J. Williams, ed., American Chemical Society, Washington, D.C. (1983); D. J. Williams, Angew. Chem., Int. Ed. Engl., Vol. 23, 690 (1984); "Nonlinear Optical Properties of Organic Molecules and Crystals", Vol. 2, D. S. Chemla, et al., ed., Associated Press, Orlando, Fla. (1987).
Although a large number of organic and inorganic materials capable of SHG have been found since Franken's discovery, an intense search continues. Through many years of research, it is now believed that an organic molecule having a conjugated x electron system or a low-lying charge transfer excited state often has a large second order polarizability, .beta.. Many molecules with large .beta. have been discovered based on these principles. However, many of these molecules have vanishing .chi..sup.(2) because of their unfavorable centrosymmetric crystal structures and therefore have no practical use. To this date, there is no absolute way of predicting whether a molecule can crystallize in a noncentrosymmetric structure.